1. Field of the Invention
The present invention relates to a regulator, and more specifically relates to a switching regulator.
2. Description of Related Art
Switching regulators are widely used to provide regulated voltage and current. Considerable ongoing research is focused on making regulators more efficient for saving power. A switching regulator typically includes a control circuit, a switch and a transformer. The control circuit is applied to sense the output voltage and/or the output current of the switching regulator, and generate a control signal to control the switch and regulate the output voltage and/or the output current of the switching regulator.
FIG. 1 shows a circuit diagram of a traditional switching regulator. The traditional switching regulator includes a transformer 10 having a primary winding NP, a secondary winding NS and an auxiliary winding NA. A terminal of the primary winding NP is coupled to a positive supply rail VIN. A switch 11 is connected from another terminal of the primary winding NP to a negative supply rail (a ground) through a resistor 12. A control circuit 25 is coupled to the switch 11 to control the switch 11 for switching the transformer 10 and regulating the output voltage and/or the output current of the switching regulator. A terminal of the secondary winding NS connects a rectifier 13. A filter capacitor 14 is coupled between the rectifier 13 and another terminal of the secondary winding NS. Energy is stored into the transformer 10 when the switch 11 is turned on. The energy stored in the transformer 10 is discharged to the output of the switching regulator through the secondary winding NS once the switch 11 is off. Meanwhile, a reflected voltage VAUX1 is generated at the auxiliary winding NA of the transformer 10.
                                          V            O                    +                      V            F                          =                              N            NS                    ×                                    ⅆ              Φ                                      ⅆ              t                                                          (        1        )                                          V                      AUX            ⁢                                                  ⁢            1                          =                              N            NA                    ×                                    ⅆ              Φ                                      ⅆ              t                                                          (        2        )            In accordance with equations (1) and (2), the reflected voltage VAUX1 can be expressed as
                              V                      AUX            ⁢                                                  ⁢            1                          =                                            N              NA                                      N              NS                                ×                      (                                          V                O                            +                              V                F                                      )                                              (        3        )            where NNA and NNS are respectively the winding turns of the auxiliary winding NA and the secondary winding NS of the transformer 10; VO is the output voltage of the switching regulator; VF is a forward voltage drop of the rectifier 13; the φ is magnetic flux, φ=B×Ae (B is flux density, Ae is the core cross-section of the transformer 10).
The control circuit 25 comprises a supply terminal VDD and a ground terminal GND for receiving power. A voltage divider includes a resistor 15 and a resistor 16 connected between the auxiliary winding NA of the transformer 10 and the negative supply rail. A voltage detection terminal VS of the control circuit 25 is connected to a joint of the resistor 15 and the resistor 16. A detecting voltage VDET1 generated at the voltage detection terminal VS is given by,
                              V                      DET            ⁢                                                  ⁢            1                          =                                            R              16                                                      R                15                            +                              R                16                                              ×                      V                          AUX              ⁢                                                          ⁢              1                                                          (        4        )            where R15 and R16 are respectively the resistance of the resistors 15 and 16.
The reflected voltage VAUX1 further charges a supplied capacitor 17 via a diode 18 to power the control circuit 25. The resistor 12 serves as a current sense device. The resistor 12 is connected from the switch 11 to the negative supply rail for converting the transformer switching current IP into a current signal VCS. A current sense terminal VI of the control circuit 25 is connected to the resistor 12 for detecting the current signal VCS. An output terminal VG of the control circuit 25 generates the switching signal VPWM to switch the transformer 10. This switching regulator is generally used to regulate output voltage and output current, but it includes several disadvantages. One disadvantage is high power consumption caused by the leakage inductor of the transformer 10. A snubber circuit includes a snubber diode 19, a snubber capacitor 20 and a snubber resistor 21 to consume the stored energy of the leakage inductor of the transformer 10 for protecting the switch 11 from a high voltage spike. Another disadvantage of this switching regulator is a poor load regulation at light load and no load. The power of the control circuit 25 is supplied from the auxiliary winding NA of the transformer 10. Therefore, the operating current of the control circuit 25 represents the load of the auxiliary winding NA. If the load at the output voltage VO of the switching regulator is lower than the load consumed by the auxiliary winding NA, then the stored energy of the transformer 10 will only be discharged to the supplied capacitor 17 through the diode 18 and the auxiliary winding NA. The rectifier 13 will remain off when the switch 11 is turned off. Therefore, the output voltage VO of the switching regulator cannot be feedback through the auxiliary winding NA. The detecting voltage VDET1 generated at the voltage detection terminal VS will be only related to the voltage of the supply terminal VDD at light load and no load situations.
Another prior art is “Primary-side controlled flyback power converter” by Yang, et al; U.S. Pat. No. 6,853,563. One principal drawback of this prior-art invention is high EMI (electric and magnetic interference). The drain terminal of the switch is directly connected to the positive supply rail VIN. A parasitic capacitor of the switch and a parasitic inductor coupled together develop a high frequency resonant tank, which produces higher EMI.
The object of the present invention is to provide a switching regulator having high efficiency and low EMI. Besides, the output voltage of the switching regulator can be accurately regulated at light load and no load.